1. Field of the Invention
The present invention relates to an exhaust system for an internal combustion engine that is equipped in an automotive vehicle, and, more particular, to an engine exhaust system that has a plurality of discrete exhaust pipes branching off from an exhaust pipe and connected to a cylinder head and an exhaust pipe.
2. Description of Related Art
Typically, a catalytic converter, that is disposed in an engine exhaust line, is desirably activated in a short period of time after a start of an engine in order to improve a catalytic conversion efficiency. In recent years, for quick activation of the catalytic converter, there has been proposed a heat insulated exhaust manifold operative to hold a temperature of an exhaust gas discharged from the engine as high as possible and then to direct the exhaust gas to the catalytic converter. Such heat insulated exhaust manifolds are known from, for example, Japanese Unexamined Patent Publication No. 11-303630 and Japanese Unexamined Utility Model Publication No. 63-54816.
The prior art heat insulated exhaust manifold disclosed in Japanese Unexamined Patent Publication No. 11-303630 comprises four double-walled discrete exhaust pipes connected at upstream ends to a cylinder head and joined at downstream ends to a double-walled chamber for collecting exhaust gases passing through the discrete exhaust pipes. This double-walled structure is somewhat troublesome to manufacture the heat insulated exhaust manifold and exerts various restraints on the configuration of the heat insulated exhaust manifold. Specifically, a bent portion of the heat insulated exhaust manifold must be large in bending radius and long in length. In other words, due to layout requirements of the engine in an engine compartment, a straight portion of the double-walled heat insulated exhaust manifold must be short in length. As a result, the double-walled heat insulated exhaust manifold provides an exhaust gas with a high resistance, so as to have an adverse influence on the output property of the engine.
The prior art heat insulated exhaust manifold disclosed in Japanese Unexamined Utility Model No. 63-54816 comprises single wall discrete exhaust pipes and a double-walled chamber for collecting exhaust gases passing through the discrete exhaust pipes. This heat insulated exhaust manifold having the double-walled chamber is focused only on lowering noises of an exhaust gas flow passing therethrough and preventing pulsations of the exhaust gas from propagating into exhaust pipes before and after the double-walled chamber.
While the heat insulated exhaust manifold is advantageous to protecting parts around the exhaust manifold from heat damage and activating quickly a catalytic converter in an exhaust line, the heat insulated exhaust manifold is apt to be heated by a hot exhaust gas passing therethrough and therefore to be filled with heat. Accordingly, the heat insulated exhaust manifold is necessary to have a heat releasing mechanism. If releasing heat through a fitting flange of the heat insulated exhaust manifold through which the heat insulated exhaust manifold is fitted to the cylinder head, a rubber gasket (a sealing member) between the cylinder head and the cylinder cover and supplementary devices around the engine are apt to encounter heat deterioration.
There has been proposed in Japanese Unexamined Utility Model 4-91224 an exhaust manifold having a generally U-shaped fitting bracket. Specifically, the exhaust manifold for an in-line four cylinder engine comprises four discrete exhaust pipes made of steel pipes that are connected to a cylinder block through a fitting flange and a collective chambered pipe made of a casting pipe, that are divided by a casting partition into two collective chambers and connected to downstream ends of the discrete exhaust pipes so that exhaust gas streams passing through the discrete exhaust pipes merge together in the collective chambered pipe. The exhaust manifold is fitted to the engine by securing the U-shaped fitting bracket bolted to the collective chambered pipe to a fitting boss of a cylinder head of the engine. As the partition is also made of a casting as well as the collective chambered pipe and is thick, it does not make noises resulting from engine vibrations.
When using a steel pipe in place of the casting pipe for the collective chambered pipe in order to reduce the weight of the collective chambered pipe, the partition makes vibrations resulting from engine vibrations, so as to make noises. In addition, as the U-shaped fitting bracket is fitted to the collective chambered pipe by fastening bolts into bolt holes formed in bosses of the collective chambered pipe and also to the boss of the cylinder block, it is hard to absorb thermal expansion of the exhaust manifold due to a hot exhaust gas.
It is an object of the present invention to provide an exhaust system having a heat insulated exhaust manifold that can provide an engine with an improved output characteristic and gain improved heat insulation effectiveness.
It is another object of the present invention to provide an exhaust system having a heat insulated exhaust manifold is adapted to radiate heat from a space formed between a fitting flange of an exhaust manifold and a cylinder head and has a heat guide member operative to conduct heat toward far from the engine so as thereby to prevent a sealing member disposed between a cylinder head and a cylinder cover and supplementary devices around the engine from encountering heat deterioration.
It is still another object of the present invention to provide an exhaust system having an exhaust manifold that is improved in structural rigidity by securing a partition of the collective chambered pipe to a welded joint structure of high rigidity through which a plurality of discrete exhaust pipes are joined to a collective chambered pipe.
It is still a further object of the present invention to provide an exhaust system having an exhaust manifold that is further improved in structural rigidity by supporting the exhaust manifold to the engine through a supporting bracket that is attached at or in close proximity to a welded joint structure between discrete exhaust pipes and a collective chambered pipe.
The exhaust manifold according to an embodiment of the present invention comprises a plurality of single-shell discrete exhaust pipes connected to exhaust ports formed in the cylinder head, respectively, a collective chambered pipe joined to the single-shell discrete exhaust pipes that is made up of an internal pipe shell into which exhaust gas streams passing through the single-shell discrete exhaust pipes are introduced and merge together and an external pipe shell surrounding the internal pipe shell, and spherical joint means connecting the collective chambered pipe to the exhaust pipe disposed downstream the collective chambered pipe and supported to the vehicle body. Each of the single-shell discrete exhaust pipes has a bent portion and a straight portion continuously extending from the bent portion to the collective chambered pipe. The bent portions of the single-shell discrete exhaust pipes are oriented toward a center of a straight row of cylinders so as to lay the straight portions of the single-shell discrete exhaust pipes nearly parallel to one another, and the single-shell discrete exhaust pipes and the collective exhaust pipe are joined to each other at least partly at a height of a plane including the interface between the cylinder block and the cylinder head.
The exhaust manifold thus structured can be provided with a long straight path of exhaust gas by the straight portions of the single-shell discrete exhaust pipes and the collective chambered pipe that is straight, so as thereby to reduce the resistance of exhaust gas. This results in helping the engine to raise output. In addition, the collective chambered pipe can be provided with high heat insulation effectiveness by having a length as long as possible. This results in quick activation of the catalytic converter installed in the exhaust system. The discrete exhaust pipe that is made of a single shell is easily bent.
The spherical joint between the collective chambered pipe and the exhaust pipe allows relative movement between the exhaust manifold and the exhaust pipe. This eases undesirable transmission of rolling vibrations of the engine to the exhaust pipe through the exhaust manifold.
The engine is preferably of a rear exhaust type that draws in intake air at the front side thereof and discharges exhaust gases at the rear side thereof. Owing to this engine position, the exhaust manifold is protected from exposure to the wind of speeding vehicle. This improves the high heat insulation effectiveness of the intake manifold. In addition, this makes it possible to shorten the length of the exhaust path from the upstream ends of the single-shell discrete exhaust pipes to the catalytic converter.
The exhaust manifold may be provided with a fitting flange connected to the upstream ends of the single-shell discrete exhaust pipes. The bolt holes formed in the fitting flange are arranged alternately along opposite upper and under sides of the fitting flange in a direction of the straight row of cylinders. At least one of the bolt holes that is in close proximity to the single-shell discrete exhaust pipes, specifically the bolt hole in close proximity to the single-shell discrete exhaust pipes, is located above a horizontal plane including centers of openings of the upstream ends of the single-shell discrete exhaust pipes. This arrangement of the bolt holes provides the straight portion of each of the single-shell discrete exhaust pipes with a long path of exhaust gas a sufficient space for installation work using, for example, an impact wrench. In this connection, if the bolt hole in close proximity to the single-shell discrete exhaust pipes is located below horizontal plane including centers of openings of the upstream ends of the single-shell discrete exhaust pipes, the bent portions of the single-shell discrete exhaust pipes must be long in consideration of a space for installation work using an impact wrench and the straight portions of the single-shell discrete exhaust pipes must be correspondingly shortened.
The exhaust manifold may include a catalytic converter disposed downstream from the collective chambered pipe and under the vehicle body in close proximity to the collective chambered pipe. This arrangement of the catalytic converter can get rid of the necessity of installing a catalytic converter immediately after an exhaust manifold like an exhaust system of a front exhaust engine and, as a result, can be located as close to the collective chambered pipe as possible so as to be quickly activated due to a hot exhaust gas.
The exhaust pipe may be of a double shell type that is made up of internal and external pipe shells. The double shell exhaust pipe provides improvement of hear insulation effectiveness in cooperation with the double shell collective chambered pipe.
The collective chambered pipe may be divided into two collective chambers, the first collective chamber and a second collective chamber located closer to the engine than the first collective chamber, by the partition. Exhaust gas streams passing through the single-shell discrete exhaust pipes, respectively, for the cylinders at opposite ends of the straight row of cylinders enter into the first collective chamber and merge together. Exhaust gas streams passing through the single-shell discrete exhaust pipes and for the remaining cylinders enter into the second collective chamber and merge together. The internal pipe shell of the collective chambered pipe shell extends straight in a direction of exhaust gas streams and gradually decreases in a cross-sectional area from the upstream end to the downstream end.
The collective chambered pipe having the second collective chamber located closer to the first collective chamber makes it possible to employ the single-shell discrete exhaust pipes that have the bent portions shortened in length. The use of the partition installed in the internal chambered pipe shell makes the collective chambered pipe compact as compared with using two independent internal pipe shells for dividing the interior of the collective chambered pipe into the first and second collective chambers and makes a surface area of the collective chambered pipe small. This structure of the collective chambered pipe reduces a thermal capacity, and hence radiation of heat, of the collective chambered pipe and, in addition, gradually constricts an exhaust gas stream. In the case of installing the exhaust system to, in particular, a four-cylinder engine, a compact 4-2-1 type exhaust system can be realized. The 4-2-1 type exhaust system is referred to the exhaust system, in which four exhaust gas streams passing through the four discrete exhaust pipes merge together into two exhaust gas streams and thereafter into one exhaust gas stream, is hardly affected by back pressure and exhaust gas pulsations.
The exhaust manifold may further comprise an insulation shell made up of two mating shell halves, i.e. the upper insulation shell half and the under insulation shell half, that covers the exhaust manifold, in particular at least the single-shell discrete exhaust pipes that are apt to radiate a comparatively large amount of heat as compared with the double-shell collective chambered pipe. This prevents the exhaust manifold from losing heat to the atmosphere and, therefore, protects peripheral devices and parts from heat damage. Further, this provides the exhaust manifold with high heat insulation effectiveness.
The exhaust manifold may be provided with spacer means installed in a space between the internal and external pipe shells and of the double-shell collective chambered pipe. This spacer means separates the internal pipe shell from the external pipe shell mechanically so as to allow longitudinal expansion of the internal pipe shell due to a difference of thermal expansion between the internal and external pipe shells. Further, the spacer means has supporting rigidity greater at a specified extent of its lower portion than at the remaining portion. The difference in supporting rigidity can be realized by differing at least one of material, width in the lengthwise direction of the internal or external pipe, thickness, and mesh size for the lower portion from that for the remaining portion. The exhaust manifold with the spacer means installed in the collective chambered pipe prevents the internal pipe shell, in particular the lower portion of the internal pipe shell, from causing mechanical interference with the external pipe shell due to vibrations.
According to another embodiment of the present invention, the exhaust manifold has a collective chambered pipe divided into a first collective chamber into which exhaust gas streams passing through single-shell discrete exhaust pipes for cylinders not sequentially adjoin in firing order and a second collective chambers into which exhaust gas streams passing through single-shell discrete exhaust pipes for the remaining cylinders not sequentially adjoin in firing order by a partition so that the first collective chamber is positioned farther away from the engine than the second collective chamber. The supporting bracket is secured to the exhaust manifold at a welded joint structure between the discrete exhaust pipes and the collective chambered pipe, or in close proximity to the welded joint structure at a side of the second collective chamber, so as to extend toward the engine. The partition at its top end is preferably secured to the welded joint structure that is high in structural rigidity. The partition and the supporting bracket are separately are separately located far away from each other. This structure of the collective chambered pipe prevents transmission of engine vibrations to the partition and, accordingly, from producing noises resulting from vibrations.
The supporting bracket may comprise a bracket arm and a flange tongue connected as one right-angle piece by a curved portion. The supporting bracket at the flange tongue is secured to the collective chambered pipe at the welded joint structure or in close proximity to the welded joint structure at a side of the second collective chamber. This right angle configuration of the supporting bracket is helpful in easing thermal expansion of the exhaust manifold and in absorption of engine vibrations. In addition,
The collective chambered pipe comprises the internal and external pipe shells joined and welded at their upstream ends only so as to form a space between the internal and external pipe shells and the supporting bracket is secured to the outer pipe shell in a position downstream from the welded joint structure where the external pipe shell is separated from the internal pipe shell. This joint structure of the collective chambered pipe is advantageous, on one hand, to preventing transmission of engine vibrations to the partition in the collective chambered pipe and, on the other hand, to improving structural rigidity and weld strength in addition to realizing improved heat insulation effectiveness of the collective chambered pipe that is advantageous to quick activation of the catalytic converter.
The single-shell discrete exhaust pipe may have a downstream end portion that comprises a rounded shell portion configured and a right-angle shell portion. The four discrete exhaust pipes thus configured and connected to the collective chambered pipe form four quadrant downstream end portions arranged so as to meet in configuration the internal pipe shell of the collective chambered pipe and form a cruciform space among the right-angle shell portions in which a cruciform reinforcement is welded to the right-angle portions of the discrete exhaust pipes. The cruciform reinforcement may be made up of an upper extension of the partition of the collective chambered pipe 28 and a reinforcement strip assembled crosswise. This cruciform reinforcement welded to the discrete exhaust pipes avoids a presence of a joint gap among the discrete exhaust pipes, so as to increase structural rigidity and weld strength of the welded joint structure and, in addition, prevent the discrete exhaust pipes from making mechanical interference with one another.
According to another embodiment, the exhaust manifold covered by the insulation shell made up of the upper and under insulation shell halves configured so as to leave a specified length of heat radiating clearance or space between the insulation shell and the fitting flange. The exhaust manifold thus structured may be provided with a heat guide member that conducts and releases exhaust heat from the heat radiating space toward far from the engine. The exhaust manifold provided with the heat guide member is advantageous to preventing a sealing member disposed between the cylinder head and the cylinder cover and supplementary devices around the engine from encountering heat deterioration. The heat guide member may be formed integrally with or separately from a gasket through which the exhaust manifold 1 is installed to the cylinder head of the engine.
The insulation shell halves that are joined to one another so as to partly overlap along their lengthwise sides. The joint gap between the overlapped sides of the insulation shell halves is desirably as small as possible and, more desirably, smaller than the space distance of the heat radiation space as possible. The insulation shell thus structured is advantageous to, on one hand, preventing exhaust heat from escaping from the insulation shell through the joint gap and, on the other hand, radiating the exhaust heat into the atmosphere through the heat radiating space between the insulation shell and the fitting flange. In addition, the insulation shell thus structured enhances the heat releasing effect of the heat guide member. The exhaust manifold is suitably installed to a rear exhaust type in-line multiple cylinder engine installed in the transverse direction in the engine compartment. This engine arrangement is advantageous to protecting the engine and supplementary.
The insulation shell may be formed with a plurality of ribs or beads as reinforcement mean extending in the transverse direction thereof. The insulation shell formed with the ribs or beads is preferably installed to the exhaust manifold by fastening bolts through bolt holes formed in the ribs or beads. The exhaust manifold covered by the insulation shell thus installed improves structural rigidity of the exhaust manifold.